Pipe microphone



sept. 11, 1962 KANESUKE KlSHl ETAL.

PIPE MICROPHONE 2 Sheets-Sheet 1 Filed Dec. 22, 1959 Sept 11, 1952KANESUKE Klsx-u ETAL 3,053,339

PIPE MICROPHONE 2 Sheets-Sheet 2 Filed Dec. 22, 1959 J1 mman mum vvd-Lima 6,1500 mono {reparle} Xg aou 40o 60u 1pm: 2.01m 4,0m Gpno 10.000

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w wwmm 3,953,339 Patented Sept. 11, 1962 ice 3,053,339 PIPE MCROPHNEKanesukc Kishi, Kawasaki-shi, and Kanarne Nalratsuru and Hirotakelawalrami, Tokyo, llapan, assignors to Sony Kabashiiriiraisha (SonyCorporation), Tokyo, Japan, a corporation of Japan Filed Dec. 2'2, 1959,Ser. No. 861,262 Claims priority, application .tapan Sept. '7, 1959 2Claims. (Cl. ISL-31) This invention relates to a pipe microphone, andmore particularly to a stand microphone in which an acoustic receivingcap is attached on the upper end of an acoustic transmission line whichis composed of telescopically fitted pipes and a resistance controlledtype transducer tor microphone unit is coupled to the lower end of theacoustic transmission line.

It has been well known that an acoustic transmission pipe having auniform cross sectional area at every part thereof is considered to beequivalent to an electrical transmission line having a distributedelectrical constant. Such an acoustic transmission pipe has an improvedtransmission characteristic since standing Waves in the pipe aresuppressed, if the input terminal and the output terminal of the pipeare terminated with adequate acoustical resistances. In the case ofmaking a pipe microphone by combining the acoustic pipe and a microphoneunit, it is necessary to bring a sound source to the neighbourhood ofthe sending end of the acoustic pipe in order to obtain a sound pick upeffect.

In such a case it is convenient that the acoustic pipe is composed of aplurality of pipes which are telescopically fitted one to another toadjust the length thereof.

In general such a telescopic pipe, however, is composed of an outer pipeof comparatively large diameter and an inner pipe of comparatively small`diameter so that the whole pipe has a discontinuity in diameter at theassembled end.

The discontinuity changes acoustic transmission characteristics for theworse owing to the reilection of sound from 4the discontinuity end and amicrophone having superior characteristics can not be expected with thistype of arrangement.

The basic idea of this invention is that in the acoustic transmissionline which is composed of a plurality of pipes telescopically fitted toone another, the upper pipe has a larger diameter than a lower one sothat the latter is received telescopically sliding into the former,whereby the sound reflection from the assembled end of the pipes can beminimized to improve the transmission characteristic-s.

One object of this invention is to provide a pipe microphone havingsuperior transmission characteristics in which an acoustic transmissionpipe which is composed of a plurality of telescopic pipes has a minimumreection of sound waves transmitted therethrough.

Another object of this invention is to provide a pipe microphone inwhich a mouth piece or cap for receiving sound waves from a sound sourceis connected to the upper end of a telescopic acoustic pipe and aresistance controlled type transducer or a microphone of this type isconnected to the lower end of the telescopic acoustic pipe so that theWhole acoustic transmission system has an excellent acoustic matching tothereby improve its frequency response.

A further object of this invention is to provide a pipe microphone inwhich a telescopic acoustic pipe has minimum changes in transmissioncharacteristics and losses even after adjustment of the length of thetelescopic acoustic pipe.

Another and further object of this invention is to provide a pipemicrophone in which an `acoustic pipe composed of a plurality oftelescopic pipes can be easily and smoothly adjusted to any desiredheight thereof.

A still further object of this invention is to provide a pipe microphonein which a telescopic acoustic pipe is protected by an outer pipe toprevent external sound waves from being transmitted to the telescopicacoustic pipes and the telescopic acoustic pipes are reinforced by theouter pipe so as to mount the same rigidly onto a stand.

The other objects, features and advantages of this invention will bemore apparent from the following description taken in connection withthe accompanying drawing in which:

FIG. l is a partial sectional view of a pipe microphone, by way ofexample, according to this invention.

FIG. 2 is an enlarged sectional View of a mouth cap or mouth piece forreceiving external sound Waves, connection mean-s between the mouth capand the upper end of an acoustic pipe being also shown.

FIG. 3 is an enlarged sectional view of a resistance controlled typetransducer or a moving coil type microphone preferably used in thisinvention, connection means between the microphone `and the lower end ofan acoustic pipe being also shown.

FIG. 4 is an electrical equivalent circuit of a pipe microphoneaccording to `this invention.

lFIG. 5 shows characteristic curves for illustrating the improvedlresponse of a pipe microphone according to this invention, and

FIG. 6 shows another response curve of a pipe microphone according tothe invention as compared with that of. an ordinary microphone.

`Referring to FIG. 1, an upper movable pipe 1 is airtightly fitted tothe outside of a lower stationary pipe 2 to form a telescopic acousticpipe as a whole. It is preferable to pour a small amount of lubricationoil such as grease between the contact surfaces of the two pipes 1 and 2to faciiitate the telescopic operation. The movable pipe 1 is preferablymade of a metal pipe the inner diameter of which can be selected on theorder of 6 to 8 mm., while the stationary pipe 2 is preferably made of ametal pipe the thickness of the wall of which is made very small such as0.3 mm. in order to minimize the discontinuity of the sectional area atthe junction end of the both pipes. That is, the thickness of the wallof the stationary pipe 2 is made as small as possible and the stationarypipe is coaxially and telescopically inserted into the movable pipe 1 sothat sound waves passing from the movable pipe to the stationary pipeare prevented from reiiecting at the end of the stationary pipe 2.However, by making the thickness of the pipe 2 so small, it becomesstructurally weak.

In order to reinforce the stationary pipe 2 it is sury rounded andsupported by an outer pipe 3 which also serves as an acoustical shieldto isolate the stationary pipe 2 from external sound waves. Thestationary pipe 2 and the outer pipe 3 are coaxially connected by anupper annular member 4 and a lower annular member f4 insertedtherebetween respectively at substantially the midpoint of the height ofthe stationary pipe and the lower end thereof. The upper annular member4 serves also as a stopper for the movable pipe 1 the lower end of whichabuts against the annular member and it is preferably made of softrubber for preventing vibration which might be caused by the abutment.

The upper end of the outer tube 3 is externally threaded and screwed bya dish yshaped nut member 9 through the f center hole 2t) of the bottomof which the movable pipe 1 is passed.

Between the nut 9 and the upper end of the outer pipe 3 is inserted anannular member 4 made of rubber which is tightly pressed, by the screwednut 9, onto the outer wall of the movable pipe 1 that the movable pipeis secured to the upper end of the outer tube 3 at its adjustedposition.

Although the foregoing description has been taken, by way of example, inconnection with the telescopic acoustic pipe which is composed of themovable pipe -1 and stationary pipe 2, it will be understood that asimilar acoustic pipe can be made by using more than two movable pipeswhich are connected to one another and the lowest one is, in turn,connected telescopically, as has been described in the former example,to a stationary pipe which is mounted on a stand. It should beunderstood, however7 that the thicknesses of the walls of the pipes aremade as small as possible and that the diameter of the lower pipe issmaller than that of the adjacent upper pipe which is telescoped to thelower pipe so as to minimize the sound reliection at the telescoped orassembled end of the pipes.

At the input end of the movable pipe 1 is mounted a mouth piece or mouthcap 7 which has a convexed contour for receiving sound waves. Inside themouth cap 7 is inserted an acoustic resistance pad 11 which issandwiched between two Wire nettings 12 secured to the open end of thepipe, as shown in FIG. 2.

This pad serves as an acoustic resistance at the sending end of thetransmission pipe to suppress standing sound waves in the pipe.

It is preferable to use felt or silk layers having a thickness of 1 to 2mm., as the acoustic resistance pad 11. Moreover, it is also preferablethat a dust proof metal netting 13 be attached on the yfront face of thecap 7 to prevent dust from entering from the opening of the cap so asnot to spoil and deform the pad.

The output end of the stationary pipe 2 is acoustically and closelycoupled with a coupling pipe 6 of an electroacoustic transducer unit Tcontained in a housing 5. One part of the coupling pipe 6 may be formedby a connector 30 provided with a flange 31 which is mounted on andsecured to the stand 3 by means of screws 40.

The upper part of the connector 3() is screwed into the lower end of asocket 32;. To the upper end of the socket 32 is screwed the lower endof the outer pipe 3, at the same time the lower end of the stationarypipe 2 is in registry with the center hole 33 of the socket 32 whichalso communicates with the pipe 6.

The lower end of the connector Sti is screwed to the upper end of thecore member of the transducer unit and the other part of the couplingpipe 6 is formed by the pole piece 34. Thus an acoustic transmissionpipe is constructed by including the cap 7, telescoped pipe 1,connection parts thereof, 4stationary pipe 2, coupling pipe 6 and thepole piece 34.

The sound transmission in the acoustic pipe is carried out through thecap 7 to which sound waves from a sound eld are received, the telescopedpipe 1, stationary pipe 2, coupling pipe 6 of the transducer unit T andnally the sound waves will arrive at a diaphragm 14.

It will be appreciated that a so-called resistance controlled typeelectro-acoustic transducer is well adapted as a transducer unit whichis used for such a pipe microphone arrangement. In the transducer unitof this type, the diaphragm thereof is mainly controlled by a dampingresistance which acts as a termination resistance of a pipe end when thediaphragm is acoustically coupled with the acoustic pipe. The dampingresistance cooperates with a sending end resistance or pad 11 tosubstantially suppress standing waves in the pipe to a nullity. As thetransducer unit of this type, a moving coil type, moving ribbon type orthe like may be used.

FIG. 3 shows a cross sectional view of a moving coil type transducerunit by way of example. The sound waves pass through the coupling pipe 6to the terminal opening of the pipe and vibrate the diaphragm 14 whichis disposed in closely coupled relationship with the terminal opening.In order to vibrate the diaphragm with substantially the same intensityin the high frequency range,

. it is necessary that the gap g1 formed between the terminal opening ofthe coupling pipe 6 and the central spherical surface of the diaphragmis reduced to as low a value as possible. An acoustic resistance pad 16is provided in the neighbourhood of the back side of the diaphragm. Thepad 16 is preferably made of a felt sheet or the like and sandwiched bytwo metal plates 17 to form a spherical surface the peripheral edge ofwhich is secured to the supported edge of the diaphragm. A moving coil15 is attached to the peripheral edge of the diaphragm and suspended inthe air gap of a strong magnetic field which is established by apermanent magnet 18. In the transducer unit thus manufactured, airmolecules which are compressed or expanded by the sound pressure willvibrate the diaphragm through the coupling pipe 6, so that air moleculesexisting at the back side of the diaphragm are also compressed orexpanded. The inner and the outer metal plate 17 are respectivelyprovided with a plurality of holes h1 and h2 so that the unit willbreathe in such a manner that air molecules at the back of the diaphragmescape through the holes h1 of the inner plate, the porous layer of theacoustic resistance pad 16 made of felt and the holes h2 of outer plateto the inner atmosphere of a sealed vessel 19 of the unit. The sealedvessel 19 serves to shield the unit acoustically so that extraneoussound waves are prevented from entering into the back side of thediaphragm 14 to vibrate the same and only the sound waves received fromthe mouth cap 7 are transmitted through the pipes to arrive at the frontof the diaphragm 14. The diaphragm 14 is so damped by the pad 16 thatthe inherent vibration of the diaphragm is suppressed with the resultthat the diaphragm can operate in response to the sound pressure in thecoupling pipe 6. The pad 16 `serves as a terminal matching resistancefor the acoustic pipe so that there are a minimum number of standingwaves in the acoustic pipe and the entire acoustic transmission systemwill respond equally for every frequency of the sound waves.

Thus a substantially constant velocity vibration of the moving coil 15will be produced for a constant sound pressure in the operatingfrequency band so that a constant induced voltage can be obtained at theterminals of -the lead wires 1t).

FIG. 4 is an electro-acoustic equivalent circuit of a pipe microphonethus obtained, in which:

F=Vibrornotive force P A. given by a sound pressure P to the opening ofthe mouth having an effective area A.

rA=Sending end acoustic resistance which is given by the resistance pad11 of the cap to the acoustic pipe.

m0=Equivalent mass of the acoustic pipe upon the dian phragm. m1,m2,m3mn=Equivalent mass of the acoustic pipe at higher order resonance.

C1, C2, C3 Cn=Equivalent compliance of the acoustic pipe at higher orderresonance.

r0,r1,r2 rn=Equivalent acoustic resistance of the pipe. U :Resistancecontrolled type electro-acoustic transducer unit.

r'B=Equiva1ent acoustic resistance or output resistance, for examplegiven by the pad 16, at the receiving end of the acoustic pipe throughthe diaphragm.

The equivalent circuit shows that the acoustic pipe is terminated by theresistance I'A at the sending end and A the resistance rB at thereceiving end.

FIG. 5 shows several curves for various conditions of Y the acousticpipe.

The curve a shows a frequency response of the electromotive force at theoutput terminals of an electro-acoustic f transducer unit at a constantsound pressure when rA m0 of the pipe and the compliance of thediaphragm; a2 the fundamental standing wave determined by the length ofthe pipe. The fundamental frequency fome/1l, where C=velocity of sound,l=the length of the pipe. Curves a3, a4 -respectively show standingwaves of higher order at the nodal points.

In the case oi the acoustic pipe in which the pad 11 stands for theadequate acoustic resistance rA and the pad 16 stands for the resistancein the microphone unit T to match both ends of the acoustic pipe, thestanding waves of the acoustic pipe can substantially be eliminated, a1-though some degree of transmission loss occurs as shown by the curve b.But the response becomes gradually low at the higher frequency regionabove 5,000 c./s. as shown by the curve b. This is because of the factthat the mass of diaphragm 14 (including moving coil mass) is notneglected and acoustic coupling between the coupling pipe 6 and thediaphragm 14 is decreased. Moreover the transmission loss in theacoustic pipe is increased at this frequency region. In this case theresponse can be cornpensated by the diffraction effect of sound at thereceiving opening of the mouth cap 7, which causes the sound pressure torise at the front surface of the sending end of the pipe.

In fact, the diffraction of sound makes -it possible to flatten theresponse through the frequency region of 5010,000 c./s. to obtainimproved transmission characteristics. The sound pressure rise caused bythe diffraction effect depends upon the diameter D of the front surfaceof the mouth cap 7.

The curve C shows the response where the diameter D is mm. Such animproved response at comparatively higher frequencies of sound waves canbe obtained by selecting the diameter D to the range of 18 mm. to 22 mm.In order to adjust the quality at higher frequency ranges of sound,however, the diameter D is selected as large as 60 mm. that the responseh-as an ascending part at the neighbourhood of 6,000 c./s. as shown bythe curve d but the curve suddenly descends at higher frequencies.

FIG. 6 shows another response curve C0 of a pipe microphone according tothis invention as compared with `a response curve bo of an ordinarymicrophone. In this experiment, the compared microphones have the sameconditions except that the upper movable pipe of the microphoneaccording to this invention is telescopically tted to the outside of thestationary pipe which has a very thin wall as has been explained inconnection with FIG. 1, while the ordinary microphone has a movablemicrophone which is telescopically inserted into a station- 'ary pipewhich has a thicker wall. That is, the only one difference in conditionsis the telescopic method of mounting of the movable and stationarypipes. From the comparison it is seen that the response curve C0 `of themicrophone according to this invention is much superior to the curve boof the ordinary microphone. Namely, the curve co is more flat and hashigher sensitivity by substantially 6-7 db than the curve bo.

This is because mainly of the following facts:

The ordinary microphone has an appreciable air gap or chamber betweenthe upper movable pipe and the lower stationary pipe which has a largerdiameter 4and larger wall thickness in order to securely support theupper movable pipe, whereby a loose coupling occurs between the pipe andthe transducer unit and standing waves are established in the pipes.

The microphone according to this invention, on the contrary, has no suchair gap between the upper movable pipe and the lower stationary pipe,nor standing waves nor reflection of sound waves as has hereinbeforedescribed.

The advantages of the microphone realized by the acoustic pipe accordingto this invention are as follows:

(1) It is possible to receive effectively sound waves by so adjustingthe length of the telescopic pipes as to adequately :draw the cap 7towards a sound source. Moreover, lan eiective acoustic resistance isinserted into the cap 7 so that noises caused by the air ilow of breathcan be minimized when a user speak-s to the microphone.

(2) In the acoustic transmission path including the movable pipe,stationary pipe and coupling pipe, the discontinuity of the sectionalarea of the transmission path is also minimized from the sending toreceiving end thereof. Besides, irregular sound reflection at theintermediate connection of the pipe is effectively suppressed by thefact that the sectional area of the acoustic transmission pipe issubstantially uniform between sending side to receiving end according tothis invention.

A flat response with respect to frequencies is also obtained by theadequate damping action of the pipe microphone.

(3) Changes of transmission characteristics and losses are small due tothe fact that the discontinuity of the transmission path does not occurappreciably when the length of the acoustic pipe is adjusted.

(4) Lubrication oil is packed between outer movable pipe 1 and innerstationary pipe 2 so that the user does not touch the lubrication oil.

(5) The main acoustic transmission pipe is protected acoustically andmechanically by the outer pipe 3 so that a stand type microphone usingsuch `an acoustic pipe has no response to noises induced from extraneoussounds and no tendency toward deformation which would occur by forcesapplied during use.

It will be understood that many modifications and variations may beeffected without departing from the scope of the novel concepts of thisinvention.

What is claimed is:

1. A microphone assembly comprising a base, a thin walled stationarypipe extending from Said base, Said thin walled pipe having a thicknessnot substantially in excess of 0.3 millimeter, a thicker walled movablepipe having its inner periphery in sliding engagement with the outerperiphery of the stationary pipe, a cap terminating the input end ofsaid movable pipe, a iirst acoustical resistance within Said cap incommunication with the interior of said pipes, a rigid pipe extendingfrom said ibase and circumscribing said stationary pipe in spacedrelation along its entire length, a microphone diaphragm in said base incommunication with the interior of said stationary pipe, and a secondacoustical impedance on the opposite side of said diaphragm from saidpipe interior.

2. A microphone assembly comprising a base, a thin walled stationarypipe extending from said base, -said thin walled pipe having a thicknessnot substantially in excess of 0.3 millimeter, a thicker walled movablepipe having its inner periphery in sliding engagement with the outerperiphery of said stationary pipe, a cap terminating the input end ofsaid movable pipe, a first acoustical resistance within said cap incommunication -with the interior of said pipes, a rigid pipe extendingfrom said base and circumscri'bing said stationary pipe in spacedrelation along its entire length, a resilient collar extending betweensaid stationary pipe and said rigid pipe intermediate the ends of saidStationary pipe, said collar being positioned to serve as an abutmentfor limiting travel of said movable pipe, a microphone diaphragm in saidbase in communication with the interior of said stationary pipe, and asecond acoustical impedance disposed within said base on the oppositeside of said diaphragm from said pipe interior.

References Cited in the le of this patent UNITED STATES PATENTS2,566,094 Olson et al. Aug. 28, 1951 2,717,932 Rackham et al. Sept. 13,1955 2,852,088 Wood Sept. 16, 1958

